One type of drug delivery device known in the art is an autoinjector, which contains a medical, therapeutic, diagnostic, pharmaceutical or cosmetic compound (drug) before it is administered, and which is used to administer the compound through the skin of the patient via a hollow needle. Autoinjectors may be used by the patient themselves or by a different user, and are also used to administer drugs to animals.
Autoinjectors are typically used because they reduce the amount of training and effort needed by a user compared with that needed for a syringe, by automating either or both processes of inserting the needle into the patient and expelling the drug through the needle. They can also reduce the fear of injection by hiding the needle from the patient.
Some autoinjectors use a single spring to provide the motive power to both insert the needle into the patient and deliver the drug. Examples of this approach include the EpiPen autoinjector from Meridian and the Humira autoinjector from Abbot.
Autoinjectors typically include a pre-filled drug container to which a needle a fixed. In operation, the drug container is first driven forward to insert the needle into the patient and thereafter a plunger is driven through the drug container to eject the drug through the needle and into the patient. A single stored energy source, such as a spring may be used to provide the motive force for both needle insertion and drug injection, or separate stored energy sources may be provided.
Where an autoinjector includes only one spring to provide the force to drive both functions, the force that the spring provides for one of the functions may be higher than needed, to enable the spring to provide sufficient force for the other function. Advantageously the two functions happen one after another rather than simultaneously in order that the drug is delivered only after the needle is correctly positioned. Because the force provided by a spring typically reduces as the spring delivers energy, the spring inevitably provides a higher force for driving the first function, i.e. needle insertion, than for the following function i.e. drug delivery, whether or not this is desirable. The strength of the spring is determined by the requirement for the spring to be able to provide sufficient force and energy at every point during the drug delivery process. This often means that much higher force than is needed or desirable is provided during the needle insertion phase.
Where two springs are provided, complicated interlock mechanisms are required to ensure the correct sequence of operation and the number of layers of components required can become large and difficult to manufacture.
It is an object of the present invention to provide an autoinjector of reduced complexity, size, cost and operating noise.